Xerophytic Adaptations of Bryophytes. 187 
compensate for the loss of assimilating surface the nerve has 
become prolonged and adventitious tissues (superior and inferior 
laminae) have been formed on it. 
The sheathing leaf is also of service in providing protection 
against dryness, not only by the decrease of evaporation owing to 
the sheathing base of the leaf, but chiefly owing to the small reser¬ 
voirs formed between the stem and the sheathing portion, the 
latter consisting of colourless, firm and extended cells. The 
sheathing leaf is shown in Bartramia ithyphalla, Dicranum uncinatum, 
Swartzia inclinata, and some species of Dicranella, but attains 
its greatest development in Polytrichum. 
The apex of the shoot forms a cup-like rosette of leaves in 
some plants (e.g. Encalypta). In the damp condition the rosette 
opens out, takes in dew or rain-drops, but closes up on the return 
of dryness. The leaf-cups of Tetraphis are gemma-bearing rather 
than water-storing, and many similar rosettes serve for holding 
antheridia and archegonia. 
(2). Water-Storing Cells. 
(a) Inflated and Hyaline Cells of the Leaf-base serve as water 
reservoirs, as well as being of importance in connection with the 
mechanics of the position-changes in leaves (8). The walls of 
these cells are thin and often perforated by small pores (especially 
when thc : r walls are thickened as in Encalypta), so that water may 
be absorbed quickly; they then become swollen out and act as 
water-storers. Owing to being concealed by the overlapping parts 
of other leaves, their usual thin-walled nature is of no disadvantage 
to them during dry weather, as they are screened from the effects 
of wind and sun. The leaves of these bryophytes are often 
appressed or curled when dry, becoming more or less patent on 
being moistened so that they retain any water running down to 
them. The amount and position of this tissue varies in different 
plants. In Polytrichum it fills up the leaf-base; in Weisia, Tricho- 
stomum, Rhabdoweisia and many liverworts a similar position is 
occupied but not so high up; in Tortula and Encalypta it lies in the 
middle portion of the leaf-base, whilst in Ulota spp., Pleurochcete 
squarrosa, Trichostomum davovirens, T. nitidum, T. fragile, T. incli- 
natum, T. tortuosum, and Brachythecium albicans, the hyaline cells 
extend up the margin of the leaf. 
True auricles, or hyaline-celled distensions of the leaf-base 
found in species of Campylopus, Dicranodontium, Dicranum, Dicra - 
